Carla Babb at the Pentagon, Nike Ching at the State Department, National Security correspondent Jeff Seldin and Extremism Watch reporter Sirwan Kajjo contributed to this report.

WHITE HOUSE - U.S. President Donald Trump sent out a stern but conflicting warning Monday saying that U.S. troops would step aside for an expected Turkish attack on the Kurds, but he threatened to destroy the Turkish economy if they went too far.

"If they do anything outside what we think is humane," Trump told reporters Monday afternoon, "they could suffer the wrath of an extremely decimated economy."

The president's Republican allies expressed outrage at the prospect of abandoning Syrian Kurds who had fought Islamic State alongside U.S. troops.

One of Trump's most loyal supporters in the Senate on Monday called the decision "shortsighted and irresponsible."

Appearing on the morning show "Fox & Friends," Republican Sen. Lindsey Graham was asked whether he supported the president's move.

"Absolutely not," he said, claiming the move was a "big win for ISIS." Graham said the Kurds in the area will align with Syrian dictator Bashar al-Assad because they'd have no choice. "So, this is a big win for Iran and Assad."

In response to a question from VOA about bipartisan concerns that Trump is effectively clearing the way for a Turkish massacre of the Kurds in Syria, a senior administration official said the U.S.'s action "is not a green light," and such assertions are "irresponsible."

Key lawmakers of both U.S. major political parties strongly condemned the president's decision.

"A precipitous withdrawal of U.S. forces from Syria would only benefit Russia, Iran, and the Assad regime," McConnell said

Pelosi called it a "deeply disturbing development that betrays our Kurdish allies who have been instrumental partners in our mission to eradicate" Islamic State.

"...It is time for us to get out of these ridiculous Endless Wars, many of them tribal, and bring our soldiers home. WE WILL FIGHT WHERE IT IS TO OUR BENEFIT, AND ONLY FIGHT TO WIN," Trump tweeted on Monday.

"Turkey, Europe, Syria, Iran, Iraq, Russia and the Kurds will now have to figure the situation out, and what they want to do with the captured ISIS fighters in their neighborhood," Trump said.

The Trump administration announced Sunday that Turkey "will soon be moving forward" with its plans to carry out an offensive in northern Syria.

"The United States Armed Forces will not support or be involved in the operation, and United States forces, having defeated the ISIS territorial 'caliphate,' will no longer be in the immediate area," the White House said, citing a Sunday phone call between Trump and Erdogan. A small number of forward-deployed U.S. troops in northern Syria have already been pulled back, according to U.S. officials.

But amid widespread criticism Trump appeared Monday to reverse what was being viewed as his greenlighting of Turkish military action in northern Syria.

Pentagon and State Department officials have been voicing opposition to any such move by Turkey.

"Any uncoordinated military operation by Turkey would be of grave concern as it would undermine our shared interest of a secure northeast Syria and the enduring defeat of ISIS," Pentagon spokesperson Commander Sean Robertson told VOA in an email.

One source described them as similar to "daily strikes" by Turkey in the area but added these likely meant to "test the waters" for more action in Syria.

A Pentagon official, speaking on condition of anonymity, told VOA that Defense Secretary Mike Esper was in contact over the weekend with the national security team (including Joint Chiefs of Staff Chairman Mark Milley, Secretary of State Mike Pompeo, National Security Adviser Robert O'Brien and the president) to discuss the situation in northern Syria.

"The Department of Defense made clear to Turkey - as did the president - that we do not endorse a Turkish operation in Northern Syria," said Assistant to the Secretary of Defense for Public Affairs Jonathan Hoffman. "The U.S. Armed Forces will not support or be involved in any such operation. "

A senior State Department official echoed that sentiment.

"We think this operation is a very bad idea. We do not think this operation will provide more security in the fight against Daesh [IS] for Turkey or for the people of the northeast," the official said.

Trump contends it is too costly to keep supporting U.S. allied Kurdish-led forces who "were paid massive amounts of money and equipment to do so. They have been fighting Turkey for decades."

The SDF says U.S. forces "have withdrawn from border areas with Turkey," and they accused the United States of not fulfilling its responsibilities under a U.S.-Turkey agreement that involved the Kurdish fighters dismantling some of their defensive capabilities near the border to allay Turkish concerns.

Turkey views the People's Protection Units (YPG), the main force within the SDF, as an extension of the Turkey-based Kurdistan Workers' Party (PKK), which has been fighting for greater rights in Turkey's Kurdish-majority southeast for decades.

The United States, however, makes a distinction between the PKK and YPG, backing the YPG-dominated SDF in the fight against Islamic State in Syria.

The SDF is holding thousands of people in detention camps in northeastern Syria, including many suspected foreign fighters who traveled from Western nations to join Islamic State.

A spokesperson for the Syrian Democratic Council, the SDF's political wing, called the Trump administration decision "ill-conceived" and that Islamic State will "become a threat to the whole world," with the questionable fate of Islamic State fighters in SDF custody becoming a "great danger" for the region.

"The situation over the ISIS detainees who are still organizing themselves while in SDF detention is not clear" according to Amjad Othman, a spokesman for the Syrian Democratic Council. "We repeatedly called for foreign states to take responsibility for their ISIS nationals. But there was no response."

The White House, in its Sunday statement, said France, Germany and other European nations have refused to take back their nationals and that the United States will not be holding them.

The United States had about 1,000 troops in Syria that have been instrumental in the fight against IS before Trump ordered a gradual withdrawal.

Former special presidential envoy for the anti-Islamic State global coalition, Brett McGurk, says Trump's decision "demonstrates a complete lack of understanding of anything happening on the ground."